This invention relates generally to balancing of helicopter rotor blades, and more particularly to employment of weights for balancing of such blades.
Helicopter rotor blades which have provisions for chordwise balance are typically balanced only with great coarseness or difficulty. Rotor blades such as those found on the Boeing Helicopter CH-46 Sea Knight are balanced chordwise only when the blades are newly manufactured or remanufactured.
The original chordwise balance condition is accomplished in manufacturer-operated whirl towers where the new blades are flown with factory "master blades" in order to provide a blade which represents a factory nominal balance condition. This nominal condition exists only on the whirl tower and is inconsequential when the rotor blade is installed in operational use on a fielded aircraft. Factors on the fielded aircraft which render the original whirl tower nominal chordwise balance of little or no importance include, but are not limited to:
1. Aerodynamic interaction with other already installed rotor blades on the fielded aircraft. PA0 2. Condition and wear of dynamic components and other rotor head related structures such as transmissions, drive shafts, and mounts. PA0 3. Condition and history of damage repair on other installed rotor blades on the fielded aircraft. PA0 a) at least one elongated rod carried within a rotor blade, PA0 b) multiple weights received and retained on the rod, the number of weights adjustable for reducing vibration during rotor rotation. PA0 1. A threaded plug which secures the weight element in association with the rotor blade and prevents environmental phenomena such as moisture and sand from entering the rotor blade. This plug is the same size and thread as the original equipment furnished with the rotor blade by its manufacturer. PA0 2. A round stainless steel rod affixed to the center of the threaded plug which provides the post upon which the individual weight elements and their retaining clamps are attached. The individual weights and retaining clamps are inserted onto the steel rod and stacked one on the other to the desired total weight required. The rod's length and diameter are configured to the available space in the rotor blade tip architecture and is aircraft-specific. PA0 3. Multiple individual circular tungsten or other dense mass metal weight elements which are assembled into a round cylinder with a central hole having a diameter slightly larger than the stainless steel rod upon which they are stacked. The diameter of the complete weight assembly is such that it can be inserted into the chordwise blade weight provision hole without binding or requiring force. The weight length and diameter of the weight assembly are configured to the available space in the rotor blade tip architecture and is aircraft-specific. PA0 4. Stainless steel retaining clamps of approximately the same diameter as the individual weight elements. The clamps have a central hole having a diameter slightly larger than the stainless steel rod upon which they are installed. The diameter of the retaining clamp is such that it can be inserted into the chordwise blade provision hole without binding or requiring force. A slit is cut through the radius of the clamp and a recessed screw hole which permits a single metal screw to draw the radial split tight, securing the clamp hard against the stainless steel rod. The retaining clamp's length and diameter are configured to the available space in the rotor blade tip architecture and is aircraft-specific. PA0 5. A stainless steel screw is inserted into the screw hole in the stainless steel clamp. Tightening of the screw draws the radial slit together to secure the clamp to the stainless steel rod.
The most severe effect of lack of chordwise balance comes to rotor blades which have been repaired and have had the balance condition altered as a result of addition or deletion of mass due to those repairs. Without accomplishing balance of the chordwise structure of the blade, the blade can never achieve an optimal dynamic balance condition and will result in the sub-optimal balance of the rotor system in which it is installed.
The standard method of dynamic rotor blade balance is that of manipulation of pitch links, addition or removal of span rotor blade weight, bending of blade trim tabs and adjustment of blade sweep. None of these methods resolves the imbalance of the rotor blades in the chord or cross axis, meaning that all balances which do not include adjustment of the chordwise mass are accomplished in a sub-optimized manner.